Recuperator



April 1956 G. SCHEFELS ET AL 2,742,269

RECUPERATOR 2 SheetsSheet 1 Filed Aug. 12, 1952 I lvvelvroes April 17, 1956 Filed Aug. 12, 1952 G. SCHEFELS ET AL RECUPERATOR 2 Sheets-Sheet 2 IN VENT'OES Gee/v4 en S H- ALF/CED SCI/A ck M, M 1 44/ ATT'OKNE'YS I portin-gfra'mes.

compensator,v abovewhich engages -;a counter-weight stresses,

he iiiveii'tio'n relates to a recuperator, in which hot low speed, though at high temperature, through a cylinderhaving a large diameter, while the gases to be heated are conducted at a speed several vtimes as great',.ithat is tosa'y t'ento thirty ttimes as great, througha narrow annular space surroundingpthe cylinder. Recuperators Uni ed States Pilteflt O p t by the reduced Width or the channel, so, that reductiori' -ga'se's ,l forf example the waste-gases of coal-dust; firing, cupola furnaces, and the like, are passed at a relatively o'f'this type, which a're primarily radiation recuperators,

have hitherto been so constructed as a cast-ironcylinder having a large number of longitudinal ribs and surrounded at a small distance by a sheet-metalcasing. T he gases tob'e heated flowed in'th e axial direction through the narrow annular space left between the cylinderand casing Such recuperators are only adapted to be used as small recuperators, because the ribbed easing, must be matte substantially wholly oftcast-iron.

lt fhas previously beefn proposed to providea fundam-entally new design according to which the inner cylinder-is. formed of sheet-metal, in given cases even f,,soft ste'eLa helical; member, guiding. the gasesto be heated helicallylthrough the annular space,gbeing welded infthe annular space. tqtheinner cylinder. Thisconstructi'on' may be, used: wherever. the cast-iron recuperator fails; Such a constructionmay also beused incombination with ac'onvection recuperaton-whichis placed to'the passages of smaller cross-section for the heating gas, so that the velocities of the heating gas are higher.

, However, it has been-found that in spite of these tundamental advantagesethis designcan also only be; used to adimited extent, and that-in recuperators through which large amounts, of gas are passed at high temperatures, numerous: disadvantages particularly distortions and craclgs occur to a considerable extent.

According to (the invention, it has been} vfound that 7 these disadvantages 1 can be overcome by relatively simple 1116338,.th6 effects of which complement each other.- These-means consist in introducing the gas'esto be heated tangentially into an; annular; and gradually narrowing channel-1 and then passing; them helically, through? guide= membe-rs'whichi are looselyinserted, orloosely attached within the annular space, to escape finally through a gradually widening channel. At the bottom, and "in some ,cases also'at L the: top, the double-walled cylinder formedbiy twoacoinzentric outer andinner cylinders is received in supportingmembers, which are secured to' the colder parts, by mean-Swot resilient members; this may be efiected by means'rof horizontally displaceable sup-' The outer casing is provided with a which counter-balances. the adjusting'forces of the compensator andssubjects the'inner cylinder 'to light tensile The advantageouswefiect of. .'this-;design. due to the factwthat: all these means;re1-ieve the inner. cylind'enof additional thermal stresses and of the harmful efiects of 2 these thermal stresses in the radial and axial directions throughout its entire length.

At the position at which the gases to be heated enter, by means of the annular channel and particularly by reason or the tapered part, care is taken that the increase in height of the channel caused by the starting pitch of the plates forming a helix, is Wholly orpartly compensated in velocity, which would involve acorrespond-inglypoor heat transmission, is avoided and thus. also local excess temperature in the innereylinder'. Furthermore, dead spaces are in this way 'avoided. In this manner it is even possible to accelerate the new of gases to be heated by reducin the eross-secti' n, so that it is possible to force the gasesflowing if; the inner cylinder automaticall'y into the boundary zone, thatis tosay, against the 7 cylinder wall, by inserting a cent'ral cover-plate, so that at the same time hot gases are prevented from escaping through the core of the inner cylinder, in which case they would not give out sufficient heat. Tests made in al'arg'e planthave shown that the total heat-transmission by'th'e recupe'rato'r is considerably increased, to an extent of ino'regthan 10%.

The middle part of the inner cylinder remains undis torted' and free from cracks, because it is no longer constri-c 'te'd by the helicalguide plates, which are loosely in} s'erte'd; or loosely attached, at a corresponding clearance,

so that the considerably hotter "cylinder is allowed to expand in the radi'al direction without becoming dented or otherwise distorted. Expansion is also possible at the bottom," because the double cylinder is mountedfresiliently,

40 'rear of the radi-ation recuperator and isoperated-with' for 'exan'iplese as to be sli'dables When he radial stresses ofthe, cylinder are it; this way overcome b'y'the interenga'gingmeans, which compensate each other, as hereinb'efore described, the, recuperator would nevertheless notbe able to meet high requirements in all cases, were it not possible according to the invention, for axial expansions to occur Without danger being inyolve'd, For this purpose, thetcompens'ator is mounted in the outer cylinder. However, this compensator as a rule requires considerable'adju'stin'g forces. These an justing forces are compensated in that a counterweight engages the double-walled'cylihder above the compensator, so thatthe inner cylinder does not utilise the adjust ing forces when it expands axially. It may even be 'subjected to light tensile stresses by the counter-weighhor forces, such as the forces of springs, replacing thecounter weight, so" that-it is possible to use heat-resistant steel, which has little hardness at the temperatures existing. It is" also possible ti) use special steels, such as high inelt'ingpoint chromium steel alloysor chromium nickel"- steel alloys, using, according to the invention furthermore, these steels at temperatures exceeding the so-callcd ,temperature of embrittlementfl In some cases, it is advant'a'geou's to ca'use an increase in the wall-temperature in these zones, that is 'tofsay in those cases in which danger wouldbe involved in using. low-melting point and-nonembrittling steel alloysat these positions, at which overlieating is unavoidable; According to the invention,- at these positions, the temperature of whichis; however,

generally within the embrittling/range, the wall-temperature must be increased so as to exceed this temperature for example to above 4755 0O with 13%-24% Cr and to above 700-750-with above 24% Cr); by reducing the speedand the coefiicient of heat transmission of theheat-absorbing means. According to the invention, at those temperature ranges in the recuperator which are outsidethe embrittlement range, the velocity of the airstream,- andthus its coefficient of heat transmission, must be made-as high as corresponds: to the thermodynamic 2,742,269 Patented Apr. 17, 1956 advantage that the part, which is particularly exposed to danger, namely the inner cylinder, is largely relieved of any additional stresses and distortions resulting therefrom. The so-called spiral recuperator thus conquers the fields of application, into which hitherto a steelrecuperator could not penetrate. a One example of a recuperator constructed according to theinvention is diagrammatically illustrated by way of example in the accompanying drawings in which:

Figure 1 is a longitudinal section of a recuperator according to the invention;

Figure 2 is a corresponding plan view; Figure 3is a section on the line AA of Figure '1; Figure 41 is a section on the line B-.-B of Figure '1; Figure 5 is a section on the line C-C of Figure 1;

Figure 6 is a sectional plan showing the sheet metal plates serving as secondary heating surfaces.

[The .recuperator is provided with an inner cylinder 1 and an outer cylinder 2, which are connected together at the top and at the bottom to form an enclosed space. The heat-radiating gases flow upwardly through the interior space 3 of the cylinder 1, in the upper part of which they are conducted towards the outer walls by the cen tral'jplate 4, as indicated by the arrows. The gases'are then passed into the vent 5. In the outer cylinder 2, is

provideda bellows type compensator 6, which allows axial thermal expansion of the inner cylinder. The adjusting forces of the compensator 6 are generated by a counter-Weight "7 mounted on a lever-arm 8, which 'is pivotable on a spindle 9, and the other end of which engages beneath claws 10, which are connected to the inner cylinder 1 by means of metal plates 11. A number of counter-weights. 7 are provided ,togeflier with corresponding lever arms so as to encircle the central axis of the recuperator. The function of the counter-weights is to support the inner cylinder 1 which has little resistance to axial collapse at high temperatures because it is made from high heat resistant nickel chrome steel.

At the bottom, the double-cylinder 1, 2 is supported on frame members or brackets 12,'which'are connected to the colder portion of the frame, floor or the like, by means of screw bolts 14 secured in a'member'14a and passing through slots 13 provided in the brackets 12. The screw bolts are tightened only to an extent sufficient to secure the general vertical position of the recuperator,

yielding however, to any thermal radial'e'xpansions by sliding inthe slots 13. V The gases to be heated flowing in at an inlet 15 pass into an annular space 16, the width of which decreases gradually, as maybe seen clearly in Figure 2. The an n'ular space 16 merges into narrow and high helical paths Jgases, heated in the meantime, then pass at the bottom into another annular space'19, which widens with decreasing height, and from which the gases escape through an outlet 20. I

In the illustrated example, the building material used for the bottom part of the inner cylinder-1 of the recuperator is chromium steel or chromium-nickel steel, whereas ordinary soft steel may be used at the top, if cold gas, and not pre-heated gas is passed in at 15. The two types of steel are so distributed that the heat-re sistant steel containing more than 13% Cr is used only at those positions in the recuperator at which its temperature is above approximately 475 C.,- whichis the embrittlement temperature. While it was'hitherto the object of the eiforts made to avoid reaching this limit,

it is intentionally exceeded in the new recuperator, so

that there-is no danger of embrittlement of the metal and fracture of the inner cylinder is thus avoided.

In those cases, in which the temperature of the entering gases to be heated falls substantially below 1000, the gas-radiation and the heat transmitted by the recuperator are much reduced. In this case gas-radiation may be considerably improved by providing secondary heating surfaces, for example in the form of a cross (Figure 3a), which may be made of brickwork or of metal plates 21. The surfaces thus added absorb heat from the gas stream both by radiation and by convection, and radiate the heat thus absorbed on to the actual heating surface, namely the cylinder 1. At first, no substantial results were obtained in this manner; due to the fact that the thickness of the layer of radiating gas is excessively reduced by this installation. According to the invention, therefore, when secondary heating surfaces are installed, the diameter of the recuperator is provided to be considerably larger than thatof similar recuperators having no secondary heating surfaces. v

When the diameter of the inner hollow cylinder is thus increased, the secondary heating surface serves also to guide the flow of gases, and to prevent reversing movements. These reversing movements occur in large crosssections when the heat-radiating gas flows slowly upwardly. The gas masses, which on their path are cooled by the wall, fall back downwardly in the middle of the wide hollow cylinder and mix at the bottom with the freshly arriving masses of hot gas. Obviously, a circulatio nof this type would considerably impair the heattransmission.

The invention can be modified in various ways. In recuperators, to which maximum requirements are made in respect of quantities and temperatures of the gas, it is advantageous to use all the means hereinbefore described simultaneously. When the requirements are not so exacting, one or the other means may be dispensed with;

The term gases is to be understood in ageneral sense, so that both individual gases and also gas-mixtures (such as air) and finally also vapours are included in this term. The recuperatoris particularly useful for the utilization of dust-containing gases, which deposit only to a slight extent and'cannot thus substantially impair the heattransmission, quite apart from the fact that the recuperator can be cleaned conveniently through appropriate cleaning openings during operation.

The recuperator may be provided in a vertical; in an inclined, or in a horizontal position. It may also be used in combination with a convection recuperator.

We claim:

l. A recuperator formed as a double-walled cylinder closed at top and bottom, a passage for the upward flow of heating .gas provided centrally of the recuperator, a

tangential inlet progressively decreasing. in cross-section at the upper end of said cylinder to increase the velocity of the gas passing through the inlet and into the annulus between the walls of the cylinder where it is heated, means mounted in said annulus to cause the gas to be heated to follow a helicalpath from the top of said cylinder to the bottom and an outlet at the bottom of said cylinder for said heated gas, said outlet being of gradually increasing cross-section to decrease the velocity of the gas, and means for supporting the inner wall of said cylinder during the axial thermal expansion thereof to prevent said inner wall of said cylinder from being subjected to stresses normally encountered in expansion.

2. A recuperator according toclaim 1, including compensator means for supporting the inner wall of said cylinder during axial thermal expansion of the inner wall of said cylinder, said compensator means comprising counter-weights secured to the ends of pivoted levers having their opposite ends in engagement with claw members secured by plates to the inner of the walls of, said'cylinderE a v 4. A recuperator according to claim 1, in which the cylinder is mounted on frame members secured to the fixed parts of the furnace by means of screw'bolts secured in slots. 1 I

5. A recuperator according toclaim' 1, in which the upper'part of the recuperator is formed of soft steel and '10 the lower part of chromium or chromium-nickel steel.

6. A recuperator according to claim 1, in which means are provided to increase gas radiation including secondary heating surfaces and in which the diameter of the References Cited in the file of this patent recuperator is increased to compensate for the reduction 15 2,555,322

in thickness of the layer of radiating gas.

UNITED STATES PATENTS Shively Oct. 15, 1889 Wainright Oct. 4, 1892 Eynon Oct. 13, 1896 Campbell June 5, 1900 Browne May 20, 1924 Baumann Apr. 14, 1925 Home et a1. June 30, 1931 Fahrenwald June 26, 1934 Schack Dec. 4, 1934 Saco Mar. 14, 1944 Yeager et a1. May 3, 1949 De Lorenzo June 5, 1951 FOREIGN PATENTS Germany June 25, 1951 

